Damage states of cut-and-cover tunnels under seismic excitation

Damage analyses of rectangular cut-and-cover tunnels are performed to define the damage states and corresponding damage indices (DIs) under seismic loading. Single, double, and triple box structures designed for metro subway systems in South Korea are used. The tunnel structures are modeled by nonlinear frame elements attached to a series of normal and shear springs to simulate the soil–tunnel interaction. Pushover analyses are performed to develop the capacity curves and to monitor the development of plastic hinges. Parallel elastic analyses are also performed to determine the elastic moments at which plastic hinges form. For each tunnel and site condition, three damage states, which are minor, moderate, and extensive, are defined in terms of number of plastic hinges that form at the corners of the tunnel structure. Each damage state is linked to the corresponding DI, which is defined as the ratio of the elastic moment to the yield moment and free-field shear strain. DI for the single box tunnel is shown to be mostly independent of the shear wave velocity of soil. The values of DIs for single, double, and triple box tunnels range from 1.0 to 2.0. It is highlighted that the proposed damage state associated with DI and shear strain provide an enhanced estimate of the seismically induced damage of box tunnels and can be easily utilized in a performance-based design